1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, which varies the cam phase of a cam relative to a crankshaft of the engine to thereby control valve timing, and more particularly to a valve timing control system of this kind, which includes an EGR device.
2. Description of the Related Art
Conventionally, a valve timing control system for an internal combustion engine has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-182566. In this control system, the cam phase of an intake cam with respect to a crankshaft is changed by changing a relative angle between an intake camshaft and its driven sprocket by a cam phase change mechanism, whereby the opening/closing timing (valve timing) of an intake valve is continuously advanced or retarded. Further, in the valve timing control system, a target cam phase is set according to detected engine speed and intake pipe absolute pressure, and then the cam phase is controlled such that the actual cam phase becomes equal to the target cam phase. For instance, in a low-load operating condition of the engine, the opening/closing timing of the intake valve is advanced by setting the target cam phase to an advanced value, to thereby increase a valve overlap between the intake valve and an exhaust valve (i.e. a time period over which the two valves are both open), whereby the amount of combustion gases remaining in a combustion chamber (i.e. an internal EGR rate) is increased to lower combustion temperature within the combustion chamber, for reduction of NOx.
However, when the above conventional valve timing control system is used together with an EGR device for recirculating exhaust gases from an exhaust system of the engine to an intake system of the same, there occurs the following problem: When the EGR device is in operation, exhaust gases are introduced as combustion gases into the combustion chamber via the intake system. However, in the above valve timing control system, the target cam phase is set using only the engine speed and the intake pipe absolute pressure as parameters, irrespective of whether the EGR device is in operation or not, and hence the proportion of the amount of combustion gases remaining in the combustion chamber resulting from a combination of an EGR rate controlled by the EGR device and the internal EGR rate controlled by the valve overlap, with respect to the whole mixture in the combustion chamber, varies depending on whether the EGR device is in operation or not. As a result, the amount of combustion gases in the combustion chamber can become excessive or insufficient, which makes it difficult to obtain a sufficient effect of reduced exhaust emissions by reduction of NOx.